The present invention relates generally to wireless data communication systems and methods, and more particularly, to a random phase shift keying technique that provides for secure modulation of wireless data communication signals.
A conventional signaling waveform for phase shift keying modulation may be expressed as                               S          i                ⁢                  (          t          )                    =                                                  2              ⁢              E                        T                          ⁢                  cos          ⁢                      (                                                            ω                  0                                ⁢                t                            +                                                φ                  i                                ⁢                                  (                  t                  )                                                      )                                ,          0      ≤      t      ≤      T                                    φ          i                ⁢                  (          t          )                    =                        2          ⁢          π          ⁢                      xe2x80x83                    ⁢          i                M              ,          i      =              1        ⁢                  xe2x80x83                ⁢        …        ⁢                  xe2x80x83                ⁢        M            
where E is the average energy in the signal, T is the duration of a pulse, xcfx890 is the carrier frequency, and M is the number of signaling points in the PSK constellation.
Referring to the drawing figures, the constellation for QPSK (M=4) is shown in FIGS. 1a and 1b. More specifically, FIGS. 1a and 1b show a conventional quadrature phase shift keying (QPSK) constellation.
A fundamental component in wireless communications is the ability of a receiver to xe2x80x9crecoverxe2x80x9d the carrier, that is, to make the receive local oscillator coherent with the transmit local oscillator. The two most common classes of carrier recovery are Mth order nonlinearity and digital symbol detection.
As for Mth order nonlinearity, applying an Mth order nonlinearity to remove modulation from the incoming carrier yields the following:                                           S            i                    ⁡                      (            t            )                          =                              [                                                                                2                    ⁢                    E                                    T                                            ⁢                              cos                ⁡                                  (                                                                                    ω                        0                                            ⁢                      t                                        +                                                                  2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                        i                                            M                                                        )                                                      ]                    M                                        =                  A          ⁢                      xe2x80x83                    ⁢                                    cos              ⁡                              (                                                      M                    ⁢                                          xe2x80x83                                        ⁢                                          ω                      0                                        ⁢                    t                                    +                                      2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    i                                                  )                                      .                              
With the modulation removed, a phase lock loop may be used to track the unmodulated carrier at M times the carrier frequency. This tracked frequency is divided by M and used for demodulation.
As for digital symbol detection, digital detection of received signaling points may be used to determine whether a point is advanced or retarded with respect to an ideal point. These early/late signals are filtered and used for carrier tracking.
Using conventional phase shift keying, an unauthorized receiver can easily recover the transmitted symbols and possibly bits. Phase randomization may be used to make accurate reception of the bits more difficult, but powerful computers can often be used to crack phase randomization codes. Therefore, phase randomization schemes are usually only computationally secure.
A computer search relating to the present invention uncovered U.S. Pat. No. 4,652,838 entitled xe2x80x9cPhase Randomization to Reduce Detectability of Phase or Frequency-modulated Digital Signalsxe2x80x9d issued Mar. 24, 1987. This patent discloses a communications system for conveying information angle-modulated onto a carrier that includes a modulator which angle-modulates the data onto the carrier and which also randomly changes the phase of the carrier. Since the carrier phase is randomly changed, an unauthorized receiver or interceptor of the signal who frequency-multiplies the signal and integrates over a long period in order to reconstruct the carrier produces a carrier which has a broader bandwidth and relatively less amplitude than in the case in which the carrier phase is not randomly changed. The probability of intercept and demodulation by the unauthorized receiver is thereby reduced. The authorized receiver frequency-multiplies and integrates for the known frame interval to recover the carrier.
With regard to the present invention, while the general purpose of the technique disclosed in this patent is the same as the present invention, the implementation and effects are quite different. The technique disclosed in U.S. Pat. No. 4,652,838 randomizes the continuous waveforms where RPSK simply randomizes the transmitted symbols. The technique disclosed in U.S. Pat. No. 4,652,838 seems to adversely effect performance by causing bandwidth expansion where the present invention cause no bandwidth expansion. The technique disclosed in U.S. Pat. No. 4,652,838 requires TDMA type processing (i.e. higher than the symbol rate processing) whereas the present invention operates at the desired symbol rate (which lends itself to high rate applications). The technique disclosed in U.S. Pat. No. 4,652,838 caused performance degradation whereas the present invention performs theoretically identical to standard PSK or QAM modulation techniques. The present invention appears to be much simpler and more applicable to the most common forms of modulation used today than the technique disclosed in U.S. Pat. No. 4,652,838.
Accordingly, it would be an advantage to have an improved phase shift keying technique that provides for secure modulation of wireless data communication signals.
The present invention provides for a secure modulation technique based on the use of vector modulation and digital symbol recovery techniques developed by the assignee of the present invention. The present invention is referred to as random phase shift keying, and makes interception of standard amplitude phase keyed (APK) waveforms nearly impossible.
The present invention provides for communication architectures and methods that implement random phase shift keying. Exemplary communication architecture and methods include a transmitter and a receiver coupled by way of a communications channel. The transmitter comprises a data source coupled to a mapper that produces an APK waveform having a plurality of symbol phases. A transmitter multiplier multiplies random phase signals with the APK waveform to produce a random phase keyed output signal. A vector modulator modulates the random phase keyed output signal. A transmitter mixer mixes the modulated random phase keyed output signal with a local oscillator signal from a transmit local oscillator to produce a modulated carrier signal that is transmitted over the communication channel.
The receiver comprises a demodulator including a receiver mixer that receives and mixes the received modulated carrier signal with a local oscillator signal derived from a receive local oscillator to produce an estimate of the random phase keyed signal. The estimated random phase keyed signal is synchronously sampled. A receiver multiplier multiplies a random phase signal with the estimated random phase keyed signal to produce an estimate of the APK waveform. A carrier tracking phase lock loop processes the estimated APK waveform to track the received modulated carrier signal.